Impedance monitoring system and method

ABSTRACT

An apparatus ( 14 ) for and method of measuring impedance in a capacitively coupled plasma reactor system ( 10 ). The apparatus includes a high-frequency RF source ( 150 ) in electrical communication with an upper electrode ( 50 ). A first high-pass filter ( 130 ) is arranged between the upper electrode and the high-frequency RF source, to block low-frequency, high-voltage signals from the electrode RF power source ( 66 ) from passing through to the impedance measuring circuit A current-voltage probe ( 140 ) is arranged between the high-frequency source and the high-pass filter, and is used to measure the current and voltage of the probe signal with and without the plasma present. An amplifier ( 250 ) is electrically connected to the current-voltage probe, and a data acquisition unit ( 260 ) is electrically connected to the amplifier. A second high-pass filter ( 276 ) is electrically connected to a lower electrode ( 56 ) and to ground, so as to complete the isolation of the high-frequency circuit of the impedance measurement apparatus from the low-frequency, high-voltage circuit of the capacitively coupled plasma reactor system. A method of measuring the plasma impedance using the apparatus of the present invention is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to International Application No.PCT/US02/05112, filed on Mar. 14, 2002; which claims priority to U.S.Provisional Application Ser. No. 60/276,106, filed Mar. 16, 2001. Theentire contents of these applications is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to plasma reactor systems, and inparticular relates to a method of and system for monitoring theimpedance in a parallel-plate plasma reactor system.

2. Discussion of the Background

Ionized gas or “plasma” may be used during processing and fabrication ofsubstrates (e.g., semiconductor devices, flat panel displays and otherproducts requiring etching or deposition of materials). Plasma may beused to etch or remove material from or sputter or deposit material ontoa semiconducting, conducting or insulating surface. Creating a plasmafor use in manufacturing or fabrication processes typically is done byintroducing a low-pressure process gas into a chamber surrounding asubstrate that resides on a substrate support member, more commonlyreferred to as a “chuck.”

In a capacitively coupled plasma reactor system, an electrode connectedto an RF power source resides above the chuck. The molecules of thelow-pressure gas in the chamber are ionized into a plasma by activatingthe radio frequency energy (power) source and heating electrons once thegas molecules enter the chamber. The plasma then flows over andinteracts with the substrate, which is typically biased by providing RFpower to the chuck supporting the substrate. In this regard, the chuckserves as the lower electrode, and is sometimes referred to as a “chuckelectrode.” The plasma gas flowing over the chuck is removed by a vacuumsystem connected to the chamber.

One of the key factors that determines the yield and overall quality ofthe plasma processing is the uniformity of the plasma process at thesurface of the substrate. In a capacitively coupled plasma reactor, theprocess uniformity is affected by the design of the overall system, andin particular by the physical relationship of the upper electrode, thechuck, the plasma generation source, and the radio frequency (RF) tuningelectronics. Improvements that lead to the ability to control reactorprocess uniformity are critical to manufacturers of plasma reactors andare the focus of significant efforts.

The ability to control plasma process parameters in a capacitivelycoupled plasma reactor depends to a large degree on the propermeasurement of the plasma conditions. The plasma parameters, includingthe plasma density, the electron temperature, the impinging ion energydistribution, etc., must be monitored to produce reliable processresults for advanced plasma processing systems. Those parameters arenormally termed as internal parameters. Internal parameters can bemonitored and used as a feedback to vary the external control processparameters (“system control parameters”), such as RF power, gas flowrate, gas pressure, the RF power and frequency, DC bias, etchchemistries, etch time, electrode spacing, wafer placement, and thelike.

Because of the problem of plasma disturbance and contaminationintroduced by some plasma measurement techniques, only non-intrusiveplasma monitors are used in the semi-conductor processing industries.There are presently several different non-intrusive techniques availableto measure plasma properties. One such technique is optical emissionspectroscopy, wherein light emitted by the plasma is collected andspectrally analyzed to extract the plasma properties. However, thistechnique has some serious shortcomings, such as low measurementreproducibility of emission line intensity, and lens degradation.

Another technique involves monitoring the RF voltage and current on theelectrodes. The relative phase difference can resolve the real systemimpedance and provide useful information about the plasma parameters.However, this technique is often hindered by the small phase differenceinvolved in the measurement. The substrate and the electrodes contributea large fraction to the real system impedance, while the plasmaimpedance is usually only a small perturbation (<10%) of the totalsystem impedance. Even with this limitation, these RF monitors are stillused widely in semiconductor manufacturing, as well as by the equipmenttool manufacturers in advanced process control (APC) systems.

Some plasma parameter measurement attempts have been made in APC systemsby correlating the passive RF measurements with certain processparameters, such as the so-called equipment footprint, the etch ordeposition rate, the end-point of the pattern etch, process cleanend-point, etc., to deduce the control functions or traces and establisha correlation with the level change in the discharge impedance measuredby the passive RF measurements. However, this correlation methodrequires a large number of measurements for every individual system toobtain statistically averaged plasma characteristics.

There are other problems with known plasma measuring techniques. Forexample, certain passive conventional monitoring techniques involvemeasuring the current and voltage of the RF power provided to the upperelectrode to form the plasma. However, this technique is problematicbecause the plasma reacts to the RF power signal, which can result in achange of the plasma state. Other techniques involve the use offundamental and harmonics signals produced in the plasma to detect thestate of the plasma. However, it can be difficult to obtain meaningfulmeasurements when noise interferes with the low-amplitude RF signals.

Further, in most plasma monitoring methods, the impedance of the plasmais determined by measuring the current, voltage and the phase differencebetween the two at the fundamental frequency (or the first fewharmonics) of the RF power source. The impedance contains both imaginaryand real parts. The real part is related to the resistance R associatedwith the circuit itself (called the circuit resistance) and of theplasma (called the “plasma resistance”). The imaginary part of thesystem impedance is due primarily to the capacitance C of the plasmasheaths near the electrodes, particularly for frequencies less than theplasma discharge resonance (when the plasma impedance is purelyresistive); below which the plasma is capacitive in nature and abovewhich the plasma is inductive in nature. Therefore, at the low harmonics(i.e. 2^(nd), 3^(rd), . . . ), the complex system impedance isapproximated by Z=1/jωC+R, with 1/ωC>>R. Therein, the resistance Rmostly comprises circuit resistance. Thus, it is usually ratherdifficult to determine the real part of the system impedance due to thelarge phase angle or nearly singular argument, and the difficulty ofmeasuring thereof. Moreover, the difficulty of extracting a small plasmaresistance from a relatively large circuit resistance furtherexacerbates the problem.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to plasma reactor systems, and inparticular relates to a method of and system for monitoring theimpedance in a parallel-plate plasma reactor system.

A first aspect of the invention is an apparatus for measuring impedancein a capacitively coupled plasma reactor system having an upper andlower electrode capable of forming a plasma therebetween. The apparatusincludes a high-frequency RF source in electrical communication with theupper electrode. The high frequency source is capable of generating anelectrical probe signal of a frequency higher than the fundamentalfrequency used to form the plasma in the reactor system. A firsthigh-pass filter is arranged between the upper electrode and thehigh-frequency RF source. The role of the first high-pass filter is toblock low-frequency, high-voltage electrical signals from the upperelectrode RF power supply from passing into the circuit for theimpedance measurement apparatus. A current-voltage (IV) probe isarranged between the high-frequency source and the high-pass filter, andis used to measure the current and voltage of the probe signal with andwithout the plasma present. An amplifier is electrically connected tothe current-voltage probe, and is preferably a lock-in amplifier thatlocks onto a modulated probe signal so as to increase thesignal-to-noise ratio. A data acquisition unit, such as ananalog-to-digital converter, is electrically connected to the amplifierand stores the analog current and voltage signals in digital form. Asecond high-pass filter is electrically connected to the lower electrodeand to ground, so as to isolate the high-frequency circuit of theimpedance measurement apparatus from the low-frequency, high-voltagecircuit of the capacitively coupled plasma reactor system.

A second aspect of the invention is a method for measuring the impedancein a capacitively coupled plasma processing system having an upper andlower electrode. The method includes the step of transmitting ahigh-frequency probe signal to the upper electrode through an electricalline connected to the upper electrode. This step is performed when thereis no plasma formed between the electrodes. The next step is thenmeasuring, in the electrical line, a first current and a first voltageof the probe signal. The next step involves calculating a “no plasmapresent” impedance Z_(np) based on the first current and the firstvoltage measurements. Once these measurements and calculations areperformed, the next step involves forming a plasma between the upper andlower electrodes. The next step is then measuring a second current and asecond voltage of the probe signal passing to the upper electrodethrough said electrical line. The next step involves calculating asystem impedance Z_(sys) from the second current and second voltagemeasurements. The next step involves determining a sheath resistance ofthe plasma Z_(sheath), preferably through the use of a standard model.The last step is then calculating the plasma impedance from the relationZ_(plasma)=Z_(sys)−Z_(np)−Z_(sheath).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram of a capacitively coupledplasma system (CC system) with the impedance measuring apparatus of thepresent invention electrically connected thereto;

FIG. 2 is a close-up schematic cross-sectional diagram of a portion ofthe coaxial cable connecting the high-pass filter to the high-frequencysignal generator, which includes formed therein the high-frequency IVprobe of FIG. 1;

FIG. 3 is a plot of an amplitude-modulated probe signal that is used incombination with a lock-in amplifier to reduce the signal-to-noise ratioof the IV measurement made with the IV probe;

FIG. 4 is a flow diagram of the method steps for measuring the plasmaimpedance in a capacitively coupled plasma reactor according to a firstembodiment of the present invention that utilizes a single-frequencyprobe signal; and

FIG. 5 is a flow diagram of the method steps for measuring the plasmaimpedance in a capacitively coupled plasma reactor according to a secondembodiment of the present invention that utilizes a spectrum offrequencies for the probe signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to plasma reactor systems, and inparticular relates to a method of and system for monitoring theimpedance in a parallel-plate plasma reactor system.

With reference now to FIG. 1, there is shown a capacitively coupledplasma reactor system (hereinafter, “CC system”) 10 to which is attachedan impedance measurement system 14 of the present invention. CC system10 comprises a reactor chamber 20 having an interior region 30 capableof containing a plasma 40. CC system 10 includes a planar upperelectrode 50 with a lower surface 52 and an opposing planar lowerelectrode 56 with an upper surface 58, thereby defining parallel plateelectrodes with a space 60 therebetween in which plasma 40 is formed.Upper electrode 50 is electrically connected to an upper electrode RFpower source 66, which is electrically connected to ground 72. An upperelectrode match network 80 is arranged between upper electrode 50 andupper electrode RF power source 66. Furthermore, a voltage-current (VI)probe 82 is located in the transmission line between the output of thematch network 80 and the upper electrode 50 in order to monitor thevoltage amplitude at the fundamental RF frequency of the RF generator66. In pneumatic communication with interior region 30 (e.g., throughupper electrode 50, as shown) is a gas supply system 88 that provides anionizable gas (e.g., Argon) for the formation of plasma 40.

Upper surface 58 of lower electrode 56 is capable of supporting asubstrate 100 (e.g., a semiconductor wafer, an LCD panel or otherdevices) to be processed by plasma 40. Lower electrode 56 iselectrically connected to a lower electrode RF power source 106, whichis connected to ground 72. A lower electrode match network 116 isarranged between lower electrode 56 and lower electrode RF power source106. Substrate 100 is in electrical contact with lower electrode 56 andis thus electrically part of the lower electrode.

With continuing reference to FIG. 1, impedance measurement system 14includes a first high-pass filter 130 electrically connected to upperelectrode 50, a current-voltage (“IV”) probe 140 electrically connectedto the high-pass filter 130, and a high-frequency RF source 150electrically connected to the IV probe 140. The purpose of high-passfilter 130 is to prevent a high-voltage (e.g., 1000V) low-frequency(e.g., 13.5 to 60 MHz) electrical signal generated by upper electrode RFpower source 66 from passing through to IV probe 140 and the othercomponents of system 14.

With reference now to FIG. 2, IV probe 140 is preferably formed in arelatively thick (e.g., 1″ to 1.5″ diameter) coaxial transmission line(e.g., a 50Ω line) 160 that electrically connects high pass filter 130,high-frequency signal generator 150 and upper electrode 50. Line 160includes an inner conducting wire 162 surrounded by an insulating layer166 having an outer surface 170. Surrounding outer surface 170 is anouter conductor layer 176. A second insulating layer (not shown)surrounding outer conductor layer 176 is typically provided to shieldthe line. IV probe 140 is formed in line 160 by electrically connectinga first pair of conducting leads 190 and 192 in series to the outerconductor layer 176 thereby forming a current terminal 194 at which thecurrent passing through line 160 can be measured.

IV probe 140 further includes a second pair of conducting leads 200 and202 respectively connected to plates 208 and 210 of a capacitor 214formed within a portion of insulating layer 166 immediately adjacentouter conductor layer 176. Capacitor 214 electrically connects inparallel outer conductor layer 176 to leads 200 and 202, thereby forminga voltage terminal 220 at which the voltage passing through line 160 canbe measured.

The current terminal 194 and the voltage terminal 220 for the IV probein transmission line 160 are designed to facilitate the use of a highimpedance RF monitor, which may be, for example, a Tektronix P6245 1.5GHz 10X Active Probe manufactured by Tektronix; the output of which mayserve as the input to amplifier 250. For further details, theconstruction of the IV probe is described in pending U.S. application60/259,862, entitled ‘Capacitively coupled RF voltage probe’ (filed onJan. 8, 2001), which is incorporated herein by reference in itsentirety.

With reference again to FIG. 1, system 14 further includes an amplifier250 electrically connected to IV probe 140. Amplifier 250 may be alock-in amplifier to improve the signal-to-noise ratio. System 14 alsoincludes a data acquisition unit 260 electrically connected to amplifier250. Data acquisition unit 260 is adapted to receive the analog signalsfrom amplifier 250 and convert them to digital signals and store them indigital form. In a preferred embodiment, data acquisition unit 260 is ananalog-to-digital converter with a memory. Also included in system 14 isa process control computer 270 electrically connected to the dataacquisition unit. Computer 270 receives the digital signals from dataacquisition unit 260 and processes the signals so as to perform thenecessary calculations for determining the plasma impedance, asdescribed below. Computer system 270 is preferably in electricalcommunication with CC system 10 and the various systems and sourcedtherein, so as to be able to control and adjust one or more of thesystem control parameters while processing substrate 100. Computer 270may, in fact, be the control computer for system 10.

Impedance measurement system 14 also includes a second high-pass filter276 electrically connected to lower electrode 56 and to ground 72. Inthe case where only a single frequency probe signal is used, high-passfilter 276 may include a reactive ground circuit designed for thesingle-frequency probe signal.

In the arrangement of CC system 10 in combination with impedancemeasurement system 14 shown in FIG. 1, there are two main RF circuits:one is a low-frequency (e.g., 13.56 MHz or 60 MHz) circuit associatedwith system 14 and extends from upper electrode RF power source 66 tolower RF power source 106 and to ground 72 (or from upper electrode RFpower source 66 through lower electrode and to ground via a narrow bandfilter (low impedance path to ground) designed specifically for thelow-frequency power applied to the upper electrode). The other is aparallel high frequency (e.g., several hundred MHz) circuit associatedwith impedance monitoring measurement system 14. The role of high-passfilters 130 and 276 is to isolate the high-frequency circuit from thelow-frequency circuit. The elements common to the low- andhigh-frequency circuits are upper and lower electrodes 50 and 56(including space 60 therebetween) and substrate 100. Space 60 maycontain either a substantial vacuum or plasma 40.

Theory of Operation

By isolating the high-frequency impedance measurement circuit of system14 from the low-frequency circuit associated with CC system 10, theplasma impedance of the CC system can be accurately measured. In thepresent invention, the necessary isolation is accomplished by groundingthe lower (chuck) electrode 56 via high-pass filter 276 and passing ameasuring (probe) signal from high-frequency RF source 150 to the upperelectrode 50, while blocking with high-pass filter 130 the high-voltagelow-frequency signal (and its harmonics generated by the interactionwith plasma 40) from CC system 10. Because the plasma is generated andsustained at a much lower frequency (e.g., 13.5 MHz to 60 MHz) than themeasuring frequency (e.g., 150 MHz to 600 MHz range), the isolatingcircuitry appears to have a very high capacitive impedance, and thuswill not affect plasma 40.

The probe signal frequency is preferably located in between the adjacenthigh harmonics of the RF signal provided by upper electrode RF powersource 66 (e.g., between the i^(th) and the (i+1)^(th) harmonics, wherefor example i=10), so that the probe signal is far away from thefundamental and plasma-induced harmonic levels. In addition,synchronized detection of the probe signal (e.g., by modulating theprobe signal) can be used with active measurements to increase thesignal-to-noise ratio by orders of magnitude.

The complex impedance of CC system 10 is generally represented by therelation:Z=j(ωL−1/ωC)+R,wherein ω is the angular frequency, C is the total series capacitance, Lis the series inductance, R is the resistance, and j=(−1)^(1/2). Forexample, the capacitance C is due mainly to the plasma sheathcapacitance, approximated by C_(sheath)≈ε_(o)A/d_(s)≈200 pf for systemscapable of processing workpieces 200 mm in diameter. A is the surfacearea of the parallel plates (i.e., electrodes 50 and 56) andd_(s)=λ_(D)(2V_(o)/T)^(1/2) is the thickness of the plasma sheath, whereλ_(D) is the Debye length. The inductance of the plasma is suitablyapproximated by L≈ω_(pe) ⁻²C_(o) ⁻¹≈250 pH, where ω_(pe) is the plasmafrequency defined by ω_(pe) ²=(en_(e)/ε_(o)m), C_(o)=ε_(o) A/d is thevacuum capacitance, and d is the spacing between the parallel plateelectrodes. Furthermore, e is the charge of an electron, n_(e) is theelectron number density and ε_(o) is the permittivity of free space.

For a typical capacitive discharge, at the applied frequency ω=2πf=3.78×10⁸ rad/sec (f=60 MHz), the reactive impedance is:X=[3.78 10⁸×250 10⁻¹²−1/(3.78 10⁸×200 10⁻¹²)]=(0.1−13.2)Ω.

The CC system reactance is nearly purely capacitive. In an actual CCsystem, the serial capacitance usually makes the reactance still larger,e.g., 100Ω. This value is much larger than the CC system resistance,R˜1Ω. It would be different if the impedance were measured at a muchhigher frequency, say, ω_(m)=3.78×10⁹ rad/sec (600 MHz); where thereactance becomes, X=[3.78 10⁹×250 10⁻¹²−1/(3.78 10⁹×20010⁻¹²)]=(0.95–1.32)=−0.37Ω, which is almost at resonance. In this case,the voltage and the current are nearly in phase and the CC systemimpedance is almost entirely real. In general, with a probe signal offrequency ω_(m), X=(ω_(m)L−1/ω_(m)C) R, the CC system impedance can bemore accurately measured.

The frequency ω at which the reactive impedance becomes zero,X=(ω_(o)L−1/ω₀C)=0, is known as the geometric resonant frequency of thesystem. If the frequency of the probe signal generated by high-frequencyRF source 150 is chosen to be the resonant frequency, ω_(m)=ω_(o), thenthe CC system impedance would be purely resistive. This allows themeasurement of the plasma impedance to be done more accurately. In fact,this resonance causes the plasma sheath to oscillate at nearly anyexternal driving voltage, largely enhancing the detectability of theprobe signal. However, the resonance frequency depends on the plasmainductance as well as the sheath capacitance and is a function of theplasma density and other CC system parameters. Thus, exciting the probesignal at the exact resonance requires an active signal source with awide frequency range.

When upper electrode RF power source 66 is turned off, there is no powerin the low-frequency circuit. In this instance, CC system 10 is said tobe “cold,” with no plasma 40 formed in space 60. Thus, thehigh-frequency circuit impedance is defined by the capacitance C betweenupper and lower electrodes 50 and 56, and the resistance of theelectrodes and workpiece 100, denoted by R_(c). The measured impedance(i.e., the “no plasma” impedance Z_(np)) of the “cold” system 10 is thusgiven by:Z _(np)=1/jω _(m) C+R _(c).  (1)

When RF power source 66 is turned on and gas is introduced into interiorregion 30 of chamber 20, plasma 40 is formed in space 60 between upperand lower electrodes 50 and 56. The high-frequency circuit now includesthe plasma resistance, R_(p), the plasma inductance L_(p), and thesheath capacitance C_(s), as well as the system resistance R_(c). Thus,the measured impedance now becomes:Z _(sys)=1/jω _(m) C _(s) +jω _(m) L _(p) +R _(p) +R _(c)  (2)

Equations (1) and (2) are used to separate the plasma impedanceZ_(plasma) from that of the cold CC system. The system resistance R_(c)includes resistance from substrate 100, which can decrease during aplasma process such as etching or deposition. On the other hand, theplasma impedance depends on the system control parameters, such as theRF power, gas pressure, gas flow, plasma chemistries, and thegeometrical parameters, such as the spacing between upper and lowerelectrode 50 and 56, and the like.

In the present invention, important information about the properties ofplasma 40 are provided by measuring Z_(np) and monitoring Z_(sys) inreal time during the operation of CC system 10. On a time scale ofseconds or shorter, the system resistance R_(c) does not changesignificantly, so that a change in the system impedance Z_(sys) is duemainly to a change in the plasma impedance. The plasma impedance isgiven by:Z _(plasma) =jω _(m) L _(p) +R _(p)=ω_(m) L _(p)(j+γ/ω _(m)),  (3)wherein ω_(m) is the applied frequency and γ is the electron-neutralcollision frequency. The latter parameter depends on the amount of RFpower provided to upper electrode 50 and on the gas pressure in space60. A mapping between the complex plasma impedance Z_(plasma) and theseplasma parameters is generated and processed in process control computer270.

On a time scale of minutes or longer, the plasma impedance Z_(plasma)can be kept constant with a constant RF power and gas pressure. Thechange of the system resistance R_(c) can be obtained from thetime-dependent function of Z_(sys) (equation (3)). Particularly,end-point detection for pattern etch, which is sensitive to the decreaseof the wafer resistance, and the end-point detection for deposition,which is sensitive to the increase of wafer resistance, can be providedby measuring the system impedance Z_(sys).

As mentioned above, impedance measurement system 14 includes dataacquisition unit 260 and computer 270, which allow for scalable controlfunctions to be introduced by means of mapping of the plasma impedanceZ_(plasma) to the aforementioned system control parameters (e.g., gaspressure, electrode spacing, RF power (voltage), RF frequency, etc.).

Impedance measurement system 14 is said to be “active” because itgenerates a probe signal whose voltage and current are a function of theimpedance Z_(sys) of CC system 10, and in particular the plasmaimpedance Z_(plasma). The advantage of an active impedance measurementas compared to a passive one lies in the scalability of the controlfunctions. For example, when the spacing of the electrodes changes, theplasma impedance increases in proportion. Thus, the mapping parameterscan be scaled from the measured values for a fixed spacing, without theneed for making measurements that correlate the electrode spacing to themapping parameters, as with prior art passive monitoring systems.

FIG. 3 illustrates an example voltage signal 300 applied by the highfrequency RF voltage source 150 to the upper electrode of CC system 10versus time. A 300 MHz signal 310 is amplitude modulated at, forexample, 1 kHz within the envelope 320 shown in FIG. 3. The raw voltageand current signals output by the IV probe 140 are detected, asdescribed above, through a lock-in amplifier 250 that isolates thesignal at 1 KHz modulation 320, thus increasing the signal to noiseratio. Data acquisition unit 260 then receives the detected voltage andcurrent signals and performs the subsequent calibration anddetermination of the complex impedance.

TABLE 1 Exemplary frequencies associated with the CC system andimpedance Measurement system of the present invention Upper ElectrodeFrequency 27 MHz  60 MHz  Lower Electrode Frequency 2 MHz 2 MHz ProbeFrequency 150 MHz  300 MHz  Modulation Frequency 1 KHz  1 KHz 

Table 1 illustrates two standard example sets of frequency valuesassociated with CC system 10 and impedance measurement system 14. Thefirst set of parameters (center column) utilizes an upper electrodefrequency of 27 MHz, a lower electrode frequency of 2 MHz, and a probesignal frequency of 150 MHz with a modulation frequency of 1 KHz. Thesecond set of parameters (right column) utilizes an upper electrodefrequency of 60 MHz, lower electrode frequency of 2 MHz, probe signalfrequency of 300 MHz with a modulation frequency of 1 KHz. The amplitudemodulation (AM) is locked to the local oscillator frequency of (lock-in)amplifier 250 to obtain a large signal-to-noise ratio.

Method of Operation, First Embodiment

With reference now to FIG. 4 and flow diagram 600 therein, and also toFIG. 1, a method of measuring the impedance of plasma 40 in CC system 10using impedance measurement system 14 and single-frequency samplingaccording to a first embodiment of the present invention is nowdescribed. In this first embodiment, high-frequency RF source 150 needonly be capable of generating a single frequency, e.g., 150 MHz or 300MHz.

In the first step 601, upper electrode power source 66 is turned off sothat there is no plasma 40 formed in space 60 between upper and lowerelectrodes 50 and 56. In the next step 602, a high-frequency (e.g., 150MHz) signal is generated by high-frequency source 150 and transmitted toupper electrode 50 through IV probe 140 and high-pass filter 130.

Next, in step 603, the current (I) and the voltage (V) passing throughto upper electrode 50 via line 160 are measured along line 160 using IVprobe 140. The raw output voltage V and current signal I are passed toamplifier 250, amplified, and then passed along to and received by dataacquisition unit 260, which stores and calibrates the information.

In the next step 604, the value for Z_(np) is calculated from themeasured voltage V and current I signals in computer 270 using therelation: $\begin{matrix}{{Z_{np} = {\frac{{V}{\mathbb{e}}^{j{({{\omega_{m}t} + \phi_{1}})}}}{{I}{\mathbb{e}}^{j{({{\omega_{m}t} + \phi_{2}})}}} = {{{Z_{np}}{\mathbb{e}}^{j\;\Delta\;\phi}} = {{{{Re}\{ Z_{np} \}} + {j\mspace{11mu}{Im}\{ Z_{np} \}}} \approx {\frac{1}{j\;\omega_{m}C} + R_{c}}}}}},} & (4)\end{matrix}$wherein $C = \frac{ɛ\; A}{d}$(assuming negligible structure reactances), j=√{square root over (−1)},ω_(m)=2πf_(m), R_(c) is the resistance of workpiece 100 and the upperand lower electrodes 50 and 56, A is the area of the upper electrode andd is spacing between the upper and lower electrodes. Once the impedancewithout plasma Z_(np) is computed, it is stored in computer 270 forfuture use.

In the next step 605, plasma 40 is generated (“turned on”) in space 60by flowing gas into interior region 30 of chamber 20 and activatingupper electrode power source 66 to provide RF power to upper electrode50. Lower electrode RF power source 106 may also be activated to providea bias. Then, in step 606, the current I and voltage V passing throughline 160 and to the upper electrode 50 are measured using IV probe 140.The values for I and V measured in this step are passed to amplifier250, amplified, and then passed along to and received by dataacquisition unit 260, which stores and calibrates the information.

Next, in step 607, the system impedance Z_(sys) is calculated incomputer 270 using the relation $\begin{matrix}\begin{matrix}{Z_{sys} = \frac{{V}{\mathbb{e}}^{j{({{\omega_{m}t} + \phi_{1}})}}}{{I}{\mathbb{e}}^{j{({{\omega_{m}t} + \phi_{2}})}}}} \\{= {{Z_{sys}}{\mathbb{e}}^{j\;\Delta\;\phi}}} \\{= {{{{Re}\{ Z_{sys} \}} + {j\mspace{11mu}{Im}\{ Z_{sys} \}}} \approx {\frac{1}{j\;\omega_{m}C_{s}} + {j\;\omega_{m}L_{p}} + R_{p} + R_{c}}}}\end{matrix} & (5)\end{matrix}$wherein ${C_{s} = \frac{ɛ\; A}{2\; d_{s}}},$d_(s) is the plasma sheath thickness, j=√{square root over (−1)},ω_(m)=3πf_(m), L_(p)=ω_(p) ⁻² C³¹ ¹ (C=εA/d, where d is the electrodespacing) is the plasma inductance, ω_(p) is the plasma frequency,R_(p)=L_(p)γ is the plasma resistance, and γ is the electron-neutralcollision frequency. The remaining symbols are as defined above.

Next, in step 608, a third voltage measurement is acquired in order toprovide information for the determination of the sheath impedanceZ_(sheath), given by Z_(sheath)˜1/jω_(m)C_(s) wherein C_(s)=εA/2d_(s),in step 609. Here, the sheath thickness d_(s) is modeled using knowntechniques, such as the method as described in the text, Principles ofPlasma Discharges and Materials Processing, Lieberman & Lichtenburg,John Wiley and Sons, 1994. Pp. 164–166, 327–386, or in the text, Basicprinciples of the RF capacitive discharge, Rajzer, Y. P., Shneider, M.N. & Yatsenko, N. A., CRC Press. Pp. 24–27, which portion of said textis incorporated herein by reference.

However, in many of the sheath models present in the literature, namelythose listed above, an additional measurement of voltage amplitude orpeak-to-peak voltage across the parallel plate electrodes is required.Using a VI probe 82 as described in FIGS. 1 and 2 and with reference topending application No. 60/259,862, a voltage measurement is preferablymade at the upper (and lower electrode if needed), and more specificallyat a convenient location along the transmission stub through which RFpower is transferred to the upper electrode as is shown in FIG. 1. Thevoltage measurement in step 608 comprises measuring the voltageamplitude at the fundamental RF frequency of the RF generator 66 in FIG.1.

Then, in step 609, the sheath thickness d₅ and, hence, the sheathimpedance is computed. However, in order to compute the sheath thicknessknowledge of the electron density and the electron-neutral collisionfrequency are required a priori. For example, following the text in thelatter reference, the sheath thickness may be represented as$\begin{matrix}{{{d_{s}^{2}\lbrack {( {\omega_{m}^{2} - {\omega_{pe}^{2}\frac{2d_{s}}{d}}} )^{2} + {\omega_{m}^{2}\gamma^{2}}} \rbrack} = ( \frac{e\; V_{a}}{md} )^{2}},} & (6)\end{matrix}$where d is the electrode spacing and V_(a) is the electrode voltageamplitude at the fundamental RF frequency of RF generator 66 in FIG. 1.Inspection of equation (6) identifies three unknowns, namely, the sheaththickness d_(s), electron density n_(e) (or electron plasma frequency,ω_(pe) ²=(en_(e)/ε_(o)m)) and electron-neutral collision frequency γ.Therefore, in order to solve for the sheath thickness in equation (6),two additional equations are required.

Using the impedance measurements in steps 604 and 607, the real part andcomplex part of the plasma impedance Z_(plasma) can be separatelycalculated via the following relations and serve the needs for twoadditional equations above; viz.Re{Z _(plasma) }=Re{Z _(sys) }−Re{Z _(np) }≈R _(p).  (7a)andIm{Z _(plasma) }=Im{Z _(sys) }−Im{Z _(np) }−Im{Z _(sheath)}≈ω_(m) L_(p)  (7b)

Furthermore, additional equations necessary to relate the plasmainductance L_(p) and plasma resistance R_(p) to the electron density andthe electron-neutral collision frequency include $\begin{matrix}{L_{p} \approx \frac{1}{\omega_{pe}^{2}C}} & ( {8a} ) \\{and} & \; \\{{R_{p} \approx {L_{p}\gamma}},} & ( {8b} )\end{matrix}$where both relations have been obtained from the text, Principles ofPlasma Discharges and Materials Processing, Lieberman & Lichtenburg,John Wiley and Sons, 1994, pgs. 327–386.

Since three equations (i.e., sheath model equation (6), the real part ofthe plasma impedance described in equation (7a) and the complex part ofthe plasma impedance described in equation (7b)) are available for threeunknown variables (i.e., sheath thickness d_(s), electron density n_(e)and electron-neutral collision frequency γ) equations (6), (7a) and (7b)can be written as a single equation solvable for the sheath thicknessd_(s), from which the remaining variables may be computed in step 610.Equations (6), (7a) and (7b) are then combined to obtain:$\begin{matrix}{{f( d_{s} )} = {{{d_{s}^{2}\lbrack {( {\omega_{m}^{2} - {\frac{\omega_{m}}{( {{{Im}\{ Z_{sys} \}} - {{Im}\{ Z_{np} \}} + \frac{2d_{s}}{ɛ\mspace{11mu} A\;\omega_{m}}} )\frac{ɛ\; A}{d}}\frac{2d_{s}}{d}}} )^{2} + {\omega_{m}^{4}\frac{( {{{Re}\{ Z_{sys} \}} - {{Re}\{ Z_{np} \}}} )^{2}}{( {{{Im}\{ Z_{sys} \}} - {{Im}\{ Z_{np} \}} + \frac{2d_{s}}{ɛ\mspace{11mu} A\;\omega_{m}}} )^{2}}}} \rbrack} - ( \frac{e\; V_{a}}{md} )^{2}} = 0}} & (9)\end{matrix}$wherein known values (ε, d, A, e, m, ω_(m)) and measured values(Re{Z_(sys)}, Im{Z_(sys)}, Re{Z_(np)}, Im{Z_(np)}, V_(a)) aresubstituted into equation (9). This forms a numerical expressionf(d_(s))=0 such that the function f is simply dependent on d_(s).Equation (9) is a non-linear function of d_(s) and, therefore, may besolved using the most suitable non-linear (root-finding) algorithm suchas the Newton-Rhapson method or the Bisection method.

Lastly, in step 610, the equations for the plasma impedance Z_(plasma)(7a & 7b) and the calculation of the sheath thickness d_(s) are used toadjust, via computer control, one or more of the system controlparameters while processing workpiece 100. For instance, theelectron-neutral collision frequency may be derived as $\begin{matrix}{{\gamma = \frac{\omega_{m}( {{{Re}\{ Z_{sys} \}} - {{Re}\{ Z_{np} \}}} )}{{{Im}\{ Z_{sys} \}} - {{Im}\{ Z_{np} \}} + \frac{2d_{s}}{ɛ\mspace{11mu} A\;\omega_{m}}}},} & ( {10a} )\end{matrix}$and the electron density may be determined as: $\begin{matrix}{{n_{e} = {\frac{ɛ\; m}{e}\frac{\gamma}{( {{{Re}\{ Z_{sys} \}} - {{Re}\{ Z_{np} \}}} )\frac{ɛ\; A}{d}}}},} & ( {10b} )\end{matrix}$and these two parameters may serve to provide information on the plasmastate useful for process control.Method of Operation, Second Embodiment

In a second embodiment of the present invention, high-frequency RFsource 150 is capable of generating signals at multiple frequencies,e.g., over a range from about 100 MHz to 300 MHz. In this secondembodiment, a multiple frequency scanned probe signal is used to measurethe system impedance more accurately than can be done with a singlefrequency probe signal. Particularly, the probe frequency can be scannedthrough the geometric resonance, at which the reactive impedance becomesvanishing small. Thus, measurement of the real part of the systemimpedance (i.e., the system resistance) is possible in this secondembodiment.

With reference now to FIG. 5 and flow diagram 700 therein, and alsoagain to FIG. 1, a method of measuring the impedance of plasma 40 in CCsystem 10 using impedance measurement system 14 and multiple-frequencysampling according to a second embodiment of the present invention isnow described.

The first steps of the method are steps 601–604 as discussed above inconnection with the first embodiment. The only difference is that steps602–604 are performed for each of a number of probe frequencies over aspectrum of probe frequencies. Step 701 inquires whether all frequencieshave been scanned. If not, then the probe frequency from high-frequencyRF source 150 is incremented in step 702 and steps 602–604 are repeated.If all the desired probe frequencies have been scanned, then the methodcontinues to step 605, as discussed above, which involves flowing gasinto interior region 30 of chamber 20 and activating upper electrode RFpower source to provide power to upper electrode 50 so as to form plasma40 between upper and lower electrodes 50 and 56.

Next, steps 606–609, as described above, are used to calculateZ_(plasma) for a particular probe frequency. Steps 606–609 are repeatedfor each probe frequency over the spectrum of probe frequencies. Step703 inquires whether all frequencies have been scanned. If not, then theprobe frequency from high-frequency RF source 150 is incremented in step705 and steps 606–609 are repeated. If all the desired probe frequencieshave been scanned, then the method continues to step 705, where theminimum impedance Z_(plasma) from the impedance values at the variousfrequencies is ascertained. This is readily accomplished using computer270. The minimum value of Z_(plasma) determined in this mannerrepresents the plasma impedance having a maximum real component.

Lastly, the method proceeds to step 610, wherein the information aboutthe plasma impedance Z_(plasma) is used to adjust, via computer control,one or more of the system control parameters while processing workpiece100.

The many features and advantages of the present invention are apparentfrom the detailed specification, and, thus, it is intended by theappended claims to cover all such features and advantages of thedescribed apparatus that follow the true spirit and scope of theinvention. Furthermore, since numerous modifications and changes willreadily occur to those of skill in the art, it is not desired to limitthe invention to the exact construction and operation described herein.Accordingly, other embodiments are within the scope of the appendedclaims.

1. An apparatus for measuring impedance in a capacitively coupled plasmareactor system having an upper and lower electrode capable of forming aplasma therebetween when a plasma generating RF signal is coupled to atleast one of the upper and lower electrodes, comprising: a) ahigh-frequency RF source in electrical communication with the upperelectrode and capable of generating an electrical probe signal having ahigher frequency than said plasma generating RF signal; b) a firsthigh-pass filter arranged between the upper electrode and saidhigh-frequency RF source, for passing high-frequency components of theelectrical probe signal to said upper electrode and isolating said highfrequency RF source from said plasma generating RF signal; and c) acurrent-voltage probe arranged between said high-frequency source andsaid high-pass filter, for measuring the current and voltage of theprobe signal.
 2. The apparatus as claimed in claim 1, furthercomprising: an amplifier electrically connected to said current-voltageprobe.
 3. The apparatus as claimed in claim 2, further comprising: adata acquisition unit electrically connected to said amplifier.
 4. Anapparatus according to claim 3, wherein said data acquisition unit is ananalog-to-digital converter.
 5. An apparatus according to claim 2,wherein said amplifier is a lock-in amplifier.
 6. The apparatus asclaimed in claim 1, further comprising: a second high-pass filterelectrically connected to the lower electrode and to ground.
 7. Anapparatus according to claim 1, wherein said high-frequency RF sourceand said current-voltage probe are connected by a coaxial line, andwherein said current-voltage probe is formed in said coaxial line.
 8. Anapparatus according to claim 1, wherein said high-frequency RF source iscapable of generating electrical signals having different frequencies.9. An apparatus according to claim 1, further comprising: an upperelectrode RF power source separate from the high-frequency RF source andconfigured to generate said plasma generating RF signal; and afrequency-specific path to ground, wherein the frequency-specific pathto ground acts as a low impedance path to ground for the high-frequencycomponents of the electrical probe signal but as a high impedance pathto ground for power provided by the upper electrode RF power source. 10.An apparatus according to claim 1, further including a computerelectrically connected to said data acquisition unit.
 11. An apparatusaccording to claim 10, wherein said computer is also electricallyconnected to the capacitively coupled plasma reactor system.
 12. Anapparatus according to claim 1, wherein said first high-pass filterpasses electrical signals having a frequency of at least 100 MHz.
 13. Amethod for measuring the impedance in a capacitively coupled plasmaprocessing system having an upper and lower electrode, comprising thesteps of: a) ensuring no plasma exists between the upper and lowerelectrodes and transmitting a high-frequency probe signal to the upperelectrode through an electrical line connected thereto, said probesignal having a higher frequency than a plasma generating signal appliedto said plasma processing system; b) measuring, in said electrical line,a first current and a first voltage of the probe signal; c) calculatinga no-plasma-present impedance Z_(np) from said first current and saidfirst voltage; d) forming a plasma between the upper and lowerelectrodes using said plasma generating signal; and e) calculating asystem impedance Z_(sys) in the presence of the plasma.
 14. The methodas claimed in claim 13, wherein the calculating step e) comprisesmeasuring a second current and a second voltage of the probe signalpassing to the upper electrode through said electrical line.
 15. Themethod as claimed in claim 14, further comprising: measuring a thirdvoltage of the plasma generating signal passing to the upper electrodethrough said line.
 16. The method as claimed in claim 15, furthercomprising: determining a sheath thickness d_(s) and sheath impedanceZ_(sheath).
 17. The method as claimed in claim 16, further comprising:calculating the plasma electron density n_(e) and electron-neutralcollision frequency γ.
 18. A method according to claim 17, furthercomprising: adjusting at least one control parameter of the plasmaprocessing system based on the step of calculating the plasma electrondensity n_(e) and the electron-neutral collision frequency γ.
 19. Amethod according to claim 13, wherein said step b) includes the step ofblocking low-frequency electrical signals transmitted from the upperelectrode.
 20. A method according to claim 13, wherein said step b),said measuring is performed using a current-voltage probe formeddirectly in said electrical line.
 21. A method according to claim 13,further comprising: electrically connecting a high-pass filter to thelower electrode and to ground.
 22. A method according to claim 21,wherein said step b) further includes modulating said probe signal anddetecting said probe signal with a lock-in amplifier tuned to saidmodulated probe signal.
 23. A method according to claim 13, wherein saidstep b) further includes the step of transmitting said first current andsaid first voltage to a data acquisition unit and storing said firstcurrent and said first voltage therein.
 24. A method according to claim13, wherein said step b) includes the step of selecting the probefrequency to be between a harmonic of a fundamental RF frequency used tocreate the plasma.
 25. A method according to claim 13, wherein said steph) includes modeling the sheath resistance.
 26. A method according toclaim 13, further comprising: measuring the first current and the firstvoltage over a range of probe signal frequencies; and selecting aminimum value for the plasma impedance Z_(p) in the range of the probesignal frequencies.
 27. A method according to claim 26, furthercomprising: adjusting at least one control parameter of the plasmaprocessing system based on the step of selecting.